Liquid crystal display having electrostatic discharge damage prevention

ABSTRACT

In order to prevent a liquid crystal display (LCD) from being damaged by electrostatic discharge, a plurality of static voltage discharging circuits are provided within the display. The LCD includes a first and second transparent substrates provided in a manner wherein they oppose each other and are spaced apart by a predetermined distance. The first substrate has one surface on which a color filter is deposited. The color filter is entirely covered with a common electrode. The second substrate carries a plurality of pixel electrodes in matrix and a plurality of thin film transistors. Another electrode is deposited at a periphery of one surface of the second transparent substrate. A plurality of lines, for thin film transistors, including column and row lines, and are deposited on the one surface of the second transparent substrate. The plurality of lines respectively extend to the corresponding thin film transistors. The static voltage discharging circuits are respectively provided between the electrode on the periphery of the second substrate and the plurality of lines. The static voltage discharging circuit is responsive to a predetermined static voltage appearing on each of the plurality of lines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a liquid crystal display(LCD) and more specifically to a LCD which is provided with measures forshunting or discharging static electricity which may be generated duringfabricating and/or repairing the LCD.

2. Description of the Related Art

As is known in the art, LCDs are commonly used in portable computers andother small, microcomputer-controlled devices for displaying data andgraphics generated by these devices.

Before turning to the present invention it is deemed preferable tobriefly describe, with reference to FIGS. 1 to 3, a conventional LCDwhich is disclosed in Japanese Laid-open Patent Application No.4-153625.

FIG. 1 is a sectional view of a conventional active matrix type LCDwhich is generally denoted by numeral 10. FIG. 2 is a plan view of partof a TFT (thin film transistor) substrate on which various elementsincluding TFTs are disposed, while FIG. 3 is a perspective view of aliquid crystal seal provided on another transparent substrate.

The LCD disclosed in the aforesaid Japanese Patent Application isdirected to a liquid crystal seal which, in addition to confining aliquid crystal layer, is able to electrically interconnect twoelectrodes respectively provided on opposing two substrates. Other thanthis, the arrangement is well known in the art and thus, only a briefdescription thereof will be given for simplifying the disclosure.

As shown in FIG. 1, two transparent (e.g., glass) substrates 12 and 14are provided in a manner wherein they oppose each other and are spacedapart by about 10 μm (for example). Throughout the instant disclosure,the substrate 14 is referred to as a TFT substrate in that the mainelement disposed thereon is the TFTs. A liquid crystal layer 16 isconfined in a space defined by the substrates 12 and 14 along with aliquid crystal seal 18. This seal 18 is provided at the periphery ofeach of the substrates 12 and 14 and is sandwiched therebetween. A pairof polarizers 20 and 22 are respectively attached to one of the majorsurfaces of each of the substrates 12 and 14 as shown in FIG. 1. Thesubstrate 12 carries a color filter 24 on its inboard surface. The colorfilter 24 is substantially entirely covered by a common electrode 26which is then coated with an alignment layer 28.

As best shown in FIG. 2, the TFT substrate 14 supports a plurality ofbonding pads 30 and 32, an electrode 34, a plurality of pixel electrodes36 arranged in matrix, a plurality of MOS (metal oxide semiconductor)TFTs 38 respectively coupled to the corresponding pixel electrodes 36, aplurality of column (e.g. drain) lines 40, and a plurality of row lines(e.g., gate) lines 42, and a lead 44 for electrical interconnectionbetween the pad 32 and the electrode 34.

The liquid crystal seal 18 is formed, using a known screen printingtechnique, at the periphery of one major surface of the substrate 12 inthe manner shown in FIG. 3. Thereafter, the substrate 12 is aligned andpressed on the TFT substrate 14 under a high temperature ranging fromabout 150° C. to 230° C., whereby the seal 18 is secured between thesubstrates 12 and 14. It is to be noted that the liquid crystal seal 18is provided with an opening for injecting a liquid crystal material.This opening is closed or sealed when the liquid crystal layer 16 isestablished. The closed opening is indicated by numeral 45.

The liquid crystal seal 18 is generally comprised of epoxy resin and isfunctionally divided into a first portion (depicted by A) and a secondportion (depicted by B) as indicated in FIG. 3.

That is, the first portion A is pressed on the electrode 34 (FIG. 2) andincludes a plurality of electrically conductive spherical spacers 46(FIG. 1) which are embedded in the epoxy resin. Each conductive spacer46 is a plastic bead (for example) which is coated with silver, gold,etc., which is elastically deformable, and which has a diameter slightlylarger than a space between the substrates 12 and 14, in which thespacer is disposed. Therefore, the spacers 46 are able to establishelectrical connections between the electrode 34 and the common electrode26. More specifically, the reason that the conductive spacers 46 areembedded for the purpose mentioned above is that the lower surface ofthe substrate 12 is not provided with any electrode for directlysupplying electricity to the common electrode 26.

The second portion B of the seal 18 is disposed on the peripheralportion of the TFT substrate 14 in addition to the electrode 34, asindicated by dashed lines in FIG. 2. This second portion B includes aplurality of non-conductive spherical spacers 48 which are also embeddedin the epoxy resin. Each of the non-conductive spacers 48 is a plasticbead (for example), which is elastically deformable, and has a diameterslightly larger than a space between the substrates 12 and 14, in whichthe spacer is provided.

Further, in order to secure a predetermined distance between thesubstrates 12 and 14, in the region of the liquid crystal layer 16, aplurality of spacers 50 are provided in the manner shown in FIG. 1.

When the LCD 10 is installed in a device, a backlight is applied frombehind the polarizer 22 as shown in FIG. 1.

The above mentioned conventional LCD, however, does not give anyattention to preventing damage by electrostatic discharge which mayoccur during fabricating the LCD and/or repairing the same.

More specifically, as is well known in the art, a plurality of TFTsubstrates 14 are fabricated on a large single glass plate at the sametime. After the deposition of elements such as TFTs on the glass plateis finished, the glass plate is cut into individual substrates 14. Whencutting the glass plate, static electricity may be developed and storedon the substrates 12 and 14. Further, the same discussion may hold whiletransporting and handling the TFT substrates. Still further, after anLCD equipped device is handed to a user, when the top polarizer 20 isdamaged and to be replaced by a new one, undesirable static electricitymay accumulate on the substrate 12 (and 14) during the repairing. AllTFTs are extremely susceptible to gate oxide breakdown, caused byelectrostatic discharge. Therefore, it is important to consider how tocope with the damage by electrostatic discharge.

In addition to the aforesald conventional LCD disclosed in JapanesePatent Application No. 4-153625, each of Japanese Utility-modelapplication No.61-89819 and Japanese Laid-open patent application No.62-218937 discloses a liquid crystal seal which includes a plurality ofconductive spacers similar to the spacers 46. However, none of theseprovide any teaching or suggestion for preventing an LCD from beingdamaged by electrostatic discharge.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved LCD which is provided with measures for preventing damage byelectrostatic discharge.

In brief, this object is achieved by techniques wherein in order toprevent a liquid crystal display (LCD) from being damaged byelectrostatic discharge, a plurality of state voltage dischargingcircuits are provided within the display. The LCD includes a first andsecond transparent substrates provided in a manner wherein they opposeeach other and are spaced apart by a predetermined distance. The firstsubstrate has one surface on which a color filter is deposited. Thecolor filter is entirely covered with a common electrode. The secondsubstrate carries a plurality of pixel electrodes in matrix and aplurality of thin film transistors. Another electrode is deposited at aperiphery of one surface of the second transparent substrate. Aplurality of lines for thin film transistors, including column and rowlines are deposited on the one surface of the second transparentsubstrate. The plurality of lines respectively extend to thecorresponding thin film transistors. Static voltage discharging circuitsare respectively provided between the electrode on the periphery of thesecond substrate and the plurality of lines. The static voltagedischarging circuit is responsive to a predetermined static voltageappearing on each of the plurality of lines.

An aspect of the present invention resides in a liquid crystal displaycomposing: a first transparent substrate having one surface on which acolor filter is deposited; a first electrode entirely covering the colorfilter; a second transparent substrate disposed opposite said firstsubstrate and spaced therefrom by a predetermined distance, said secondtransparent substrate carrying a plurality of pixel substrates and aplurality of thin film transistors; a second electrode deposited at aperiphery of one surface of said second transparent substrate; aplurality of lines for controlling the thin film transistors, saidplurality of lines consisting of column and row lines deposited on saidone surface of said second transparent substrate and respectivelyextending to the corresponding thin film transistors; and static voltagedischarging members provided between said second electrode and each ofsaid plurality of lines, said static voltage discharging members beingresponsive to a predetermined static voltage appearing on each of saidplurality of lines.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become moreclearly appreciated from the following description taken in conjunctionwith the accompanying drawings in which like elements are denoted bylike reference numerals and in which:

FIG. 1 is a sectional view of a conventional active matrix type LCD,referred to in the opening paragraphs of the instant disclosure;

FIG. 2 is a top plan view of part of a TFT substrate of FIG. 1 on whichvarious elements, including TFTs, are disposed;

FIG. 3 is a perspective view of a liquid crystal seal provided on asubstrate of FIG. 1;

FIG. 4 is a sectional view of an active matrix type LCD embodying thepresent invention;

FIG. 5 is a top plan view of part of a TFT substrate of FIG. 4 on whichvarious elements, including TFTs, are disposed;

FIG. 6 is a sectional view of part of the TFT substrate of FIG. 5together with a member formed thereon;

FIG. 7 is a perspective view of a liquid crystal seal provided on onesubstrate shown in FIG. 4; and

FIG. 8 is a top plan view of part of a substrate, similar to that shownin FIG. 5, on which various elements, including zener diodes, aredisposed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will be described withreference to FIGS. 4-7 wherein FIGS. 4, 5 and 7 respectively correspondto FIGS. 1-3. To simplify the description of the embodiment, the likeelements referred to in FIGS. 1-3 will be represented by like referencenumerals.

In brief, the instant embodiment differs from the conventional LCD ofFIGS. 1-3 as follows. An electrode corresponding to the electrode 34 iselongated along the periphery of the TFT substrate 14 and, a pluralityof electrical paths for discharging static electricity are establishedbetween the above mentioned electrode and each of the column/row lines40 and 42.

As shown in FIGS. 4 and 5, an electrode 60 is deposited at the peripheryof the TFT substrate 14 in a manner to cross the column/row lines 40 and44 by way of an insulating layer 62 (FIG. 6). Note that the insulatinglayer 62 is omitted in FIG. 4 for the sake of simplifying the drawing.

A plurality of pairs of shunt transistors 64a and 64b are connectedbetween the electrode 60 and each of the column/row lines 40 and 42.More specifically, the shunt transistor 64a is an n-channel (forexample) MOSFET whose drain and gate are coupled to one of the columnline 40 (or row line 42) and whose source is coupled to the electrode60. On the other hand, the shunt transistor 64b is also an n-channelMOSFET whose source is coupled to one of the column line 40 (or row line42) and whose drain and gate are coupled to the electrode 60. Therefore,if static voltage developed on the column/row lines 40 and 42 (forexample) is positive and exceeds a predetermined level, a current pathis established through the shunt transistor 64a. On the contrary, ifstatic voltage developed on the column/row lines 40 and 42 (for example)is negative and below a predetermined level, a current path isestablished through the shunt transistor 64b. That is to say, thetransistors 64a and 64b constitute static electricity discharging paths.The above mentioned predetermined level is decided depending on theactual characteristics of the TFTs 38 and the transitors 64a and 64b.

Such discharging paths, however, are not limited to the above mentionedshunt transistors 64a and 64b. As an alternative a plurality of pairs ofdiodes (inclusive of zener diodes as shown in FIG. 8) may be employedfor discharging positive and negative static voltages appearing on thecolumn/row lines. Further, p-channel MOSFETs can be used for the samepurpose.

The electrode 60 is coupled to the bonding pad 32 by way of the line 44which is further coupled to an external voltage source (not shown).

Since the electrode 60 is provided at the periphery of the substrate 14,it is preferable to construct a liquid crystal seal 66, whichcorresponds to the seal 18 (FIG. 1), so as to include the electricallyconductive spacers 46 over the entire seal 66.

The liquid crystal seal 66 is formed, using a known screen printingtechnique, at the periphery of one major surface of the substrate 12 asschematically shown in FIG. 7. On the other hand, the pixel electrodes36, the TFTs 38 and the shunt transistors 64a and 64b are deposited onthe TFT substrate 14 using ITO (Indium Tin Oxide), Al, and α-Si(Amorphous Silicon) via the well known techniques such as sputtering,lithography, doping, etc. Following this step, Al or Cr is used todeposit, on the TFT substrate 14, the column/row lines 40, 42, the line44, and the bonding pads 30, 32. Further, the electrode 60 is patternedon the substrate 14 by sputtering Al or Cr. Thereafter, the substrate 12bearing the seal 66 is aligned and pressed on the TFT substrate 14 undera high temperature ranging from about 150° C. to 230° C., whereby theseal 66 is secured between the substrates 12 and 14.

Finally, a liquid crystal material is injected, through an openingprovided in the seal, in a vacuum environment into the space defined bythe substrates 12 and 14 and the seal 66. Subsequently, the opening isclosed using UV (Ultra Violet) setting resin.

It will understood that the above disclosure is representative of onlyone possible embodiment of the present invention and that the concept onwhich the invention is based is not specifically limited thereto.

What is claimed is:
 1. A liquid crystal display comprising:a firsttransparent substrate having one surface on which a color filter isdeposited; a first electrode entirely covering the color filter; asecond transparent substrate disposed opposite said first substrate andspaced therefrom by a predetermined distance, said second transparentsubstrate carrying a plurality of pixel electrodes and a plurality ofthin film transistors; a second electrode deposited at a periphery ofone surface of said second transparent substrate; a plurality of linesfor controlling the thin film transistors, said plurality of linesincluding column and row lines deposited on said one surface of saidsecond transparent substrate and respectively extending to thecorresponding thin film transistors; a static voltage discharging memberprovided between said second electrode and each of said plurality oflines, said static voltage discharging members being responsive to apredetermined static voltage appearing on each of said plurality oflines; and a liquid crystal seal provided on said second electrode in amanner to be sandwiched between said first and second transparentsubstrates, said liquid crystal seal defining a space for a liquidcrystal layer together with said first and second transparentsubstrates, said liquid crystal seal including a plurality ofelectrically conductive spacers so as to establish an electricalconnection between said first and second electrodes.